A requirement for removing particulate matter and toxic substances contained in exhaust gas discharged from internal combustion engines, a boiler and the like has grown in light of influence on environment. In particular, regulations on removal of particulate matter (hereinafter sometimes referred to as a particulate matter (PM)) discharged from diesel engines tend to be strengthen in Europe and the United States as well as Japan, and a honeycomb structure is used in a trapping filter for removing the PM.
Examples of a filter used for such a purpose include a honeycomb filter utilizing a plugged honeycomb structure including a honeycomb structure in which a plurality of cells constituting fluid channels are partitioned by porous partition walls and formed in a honeycomb-like shape by the partition walls, and plugging portions which alternately plug one opening end and the other opening end of each of the plurality of cells. According to such a honeycomb filter, an exhaust gas G1 is allowed to flow into cells from the exhaust gas inflow side end face of the filter, whereby particulates contained in the exhaust gas are trapped by the partition walls in a case where the exhaust gas G1 passes through the partition walls. In consequence, a purified gas G2 from which the particulates have been removed can be discharged from the purified gas outflow side end face of the filter.
Moreover, as a method of manufacturing the above-mentioned plugged honeycomb structure, for example, a method is suggested which includes the steps of attaching an adhesive sheet or the like to one end face of a honeycomb structure (a non-fired dried ceramic body); making holes only corresponding to cells to be plugged (plugged cells) in the adhesive sheet or the like by laser processing or the like using image processing, to form a mask; immersing, into a slurry (a ceramic slurry), the end face of the honeycomb structure to which the mask is attached to fill the honeycomb structure cells to be plugged with the slurry, thereby forming plugging portions; subjecting the other end face of the honeycomb structure to steps similar to the above steps; and drying and firing the honeycomb structure to obtain the plugged honeycomb structure (e.g., see Patent Document 1).    [Patent Document 1] Japanese Patent Application Laid-Open No. 2001-300922
However, according to the method of making the holes in the adhesive sheet and immersing, into the slurry, the honeycomb structure provided with this adhesive sheet as the mask as described above, operation time is required for leveling the slurry into which the structure is to be immersed, and this is a factor for cost increase. Moreover, the leveling is a very delicate step, and an outer peripheral portion becomes thick or thin owing to slight displacement, so that it is difficult to obtain a uniform plugging depth. A leveling accuracy matches a depth accuracy, so that considerable amounts of labor and time are required for the leveling step. Moreover, a film attached to the structure needs to be bent so that the immersed end of the structure is not made dirty, but it is not easy to appropriately bend the film.